It is known in the glass art to form multi-layer, composite, glass bodies. U.S. Pat. No. 3,673,049, granted June 27, 1972 to J. W. Giffen et al., describes composite glass bodies comprising a tensilely stressed core and a compressively stressed surface layer, whereby the physical strength of the body is enhanced. This patent further discloses a method of producing such bodies wherein glass streams are brought together in the molten state to avoid formation of defects at the interface. Such method of producing a composite glass sheet, and a glass feeder apparatus specifically designed for carrying out the method, are the subject of U.S. Pat. No. 3,582,306 granted June 1, 1971 to J. W. Giffen. U.S. Pat. No. 3,597,305, granted Aug. 3, 1971 to J. W. Giffen, describes a laminated body embodying one or more subsurface, compressively stressed layers that fortify the body against propagation of surface cracks or bruises.
The production and use of encased glass cane as a substrate for film type resistors is described in U.S. Pat. No. 3,437,974 granted Apr. 8, 1969 to J. Spiegler. U.S. Pat. No. 3,676,043, granted July 11, 1972 to J. W. Anderson et al., describes a photoflash lamp having a three layer, laminated glass envelope. The coefficient of thermal expansion of the middle glass layer is higher than that of the inner and outer glass layers so that the latter are in compression and the middle layer is in tension.
The Anderson et al. patent suggests a method of producing the composite lamp envelope wherein glass tubes are arranged in close fitting, concentric assembly, and the assembly is progressively thermally softened for redrawing as a composite tube. However, in such redrawing process, it is difficult to avoid entrapment of particles, gas bubbles, and other foreign material at the interface. The patent also suggests, but does not depict or describe, the use of a tube forming mandrel surrounded by an orifice consisting of separate rings.